A common process for manufacturing lower olefins is through pyrolytic cracking of saturated hydrocarbon feedstocks containing hydrocarbons such as ethane, propane, butane, pentane, and crude oil fractions such as naphtha and gas oil. Producers of lower olefins are always looking for lower cost hydrocarbon feedstocks that can be economically upgraded by pyrolytic cracking processes to lower olefins. One material that is of interest for the conversion to a lower olefins product is crude oil and fractions of crude oil that contain pitch, such as petroleum residuum, due to its low cost and the possibility of significantly upgrading its value through its conversion to higher valued products. While petroleum residuum is attractive from a cost standpoint, it does not make a good feedstock for pyrolytic cracking, because it does not completely vaporize in the convection section of traditional pyrolytic cracking furnaces, and the yields of high valued products are poor.
U.S. Pat. No. 4,615,795 discloses a process and system for the production of olefins from heavy hydrocarbon feedstocks such as petroleum residuum. A heavy hydrocarbon feedstock is first pretreated at high pressure and moderate temperatures followed by separation into lighter and heavier fractions. The lighter fraction is subsequently pyrolyzed to produce olefins. However, if one wanted to increase the amount of the lighter fraction by trying to increase the separation temperatures and extract more of the lighter fraction by “deep distillation”, a commercially infeasible process would be created. This is due in part because coke would form in the distillation train as a result of the high temperatures required—conventionally, such deep distillation requires a vacuum column distillation to avoid very high temperatures that promote coke formation. It would also be difficult to completely vaporize the deep distillation cuts in the convection section of a typical pyrolytic cracking furnace in an olefins plant. In addition, the heavier feed to the furnace would likely increase the amount of tar in the furnace, and increase the coking of the furnace tubes.
A recent advance in pyrolysis of crude oil and crude oil fractions containing pitch is shown in U.S. Pat. No. 6,632,351. In the '351 process a crude oil feedstock or crude oil fractions containing pitch is fed directly into a pyrolysis furnace. The process comprises feeding the crude oil or crude oil fractions containing pitch to a first stage preheater within a convection zone, where the crude oil or crude oil fractions containing pitch are heated within the first stage preheater to an exit temperature of at least 375° C. to produce a heated gas-liquid mixture. The mixture is withdrawn from the first stage preheater and the gas-liquid mixture fed to a vapor-liquid separator, followed by separating and removing the gas from the liquid in the vapor-liquid separator, and feeding the removed gas to a second preheater provided in the convection zone. The preheated gas is then introduced into a radiant zone within the pyrolysis furnace, and pyrolyzed to olefins and associated by-products. While this is an improvement in the overall process, there are still limitations in achieving higher yields of more valuable products due to coke formation in the convection section and vapor-liquid separator at increased separation temperatures needed to increase gas feed rates to the radiant section of the furnace where pyrolysis takes place. Also, increased coke formation will occur in the radiant section because the gas removed in the vapor-liquid separator will contain higher boiling fractions as the vapor-liquid separator temperature increases—i.e., with increasing vapor-liquid separator temperature more coke precursors are removed from the liquid and fed into the radiant section of the furnace.
What is needed is an improved process that permits the economical processing of a heavy hydrocarbon feedstock to produce lower olefins in higher yield, without causing unacceptable fouling or coking in the convection section, the vapor-liquid separator or the radiant section furnace tubes.